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Thu, 22 Oct 2020 05:19:26 -0400 X-MC-Unique: YuC2ea8uNjCRXVsi9pVVtQ-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 64F581006C9F; Thu, 22 Oct 2020 09:19:22 +0000 (UTC) Received: from [10.36.113.152] (ovpn-113-152.ams2.redhat.com [10.36.113.152]) by smtp.corp.redhat.com (Postfix) with ESMTP id C826360BFA; Thu, 22 Oct 2020 09:19:16 +0000 (UTC) Subject: Re: Buggy commit tracked to: "Re: [PATCH 2/9] iov_iter: move rw_copy_check_uvector() into lib/iov_iter.c" To: Greg KH Cc: David Laight , Al Viro , Nick Desaulniers , Christoph Hellwig , "kernel-team@android.com" , Andrew Morton , Jens Axboe , Arnd Bergmann , David Howells , "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" , "linux-mips@vger.kernel.org" , "linux-parisc@vger.kernel.org" , "linuxppc-dev@lists.ozlabs.org" , "linux-s390@vger.kernel.org" , "sparclinux@vger.kernel.org" , "linux-block@vger.kernel.org" , "linux-scsi@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "linux-aio@kvack.org" , "io-uring@vger.kernel.org" , "linux-arch@vger.kernel.org" , "linux-mm@kvack.org" , "netdev@vger.kernel.org" , "keyrings@vger.kernel.org" , "linux-security-module@vger.kernel.org" References: <20200925045146.1283714-1-hch@lst.de> <20200925045146.1283714-3-hch@lst.de> <20201021161301.GA1196312@kroah.com> <20201021233914.GR3576660@ZenIV.linux.org.uk> <20201022082654.GA1477657@kroah.com> <80a2e5fa-718a-8433-1ab0-dd5b3e3b5416@redhat.com> <5d2ecb24db1e415b8ff88261435386ec@AcuMS.aculab.com> <20201022090155.GA1483166@kroah.com> From: David Hildenbrand Organization: Red Hat GmbH Message-ID: Date: Thu, 22 Oct 2020 11:19:15 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.3.1 MIME-Version: 1.0 In-Reply-To: <20201022090155.GA1483166@kroah.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 22.10.20 11:01, Greg KH wrote: > On Thu, Oct 22, 2020 at 10:48:59AM +0200, David Hildenbrand wrote: >> On 22.10.20 10:40, David Laight wrote: >>> From: David Hildenbrand >>>> Sent: 22 October 2020 09:35 >>>> >>>> On 22.10.20 10:26, Greg KH wrote: >>>>> On Thu, Oct 22, 2020 at 12:39:14AM +0100, Al Viro wrote: >>>>>> On Wed, Oct 21, 2020 at 06:13:01PM +0200, Greg KH wrote: >>>>>>> On Fri, Sep 25, 2020 at 06:51:39AM +0200, Christoph Hellwig wrote: >>>>>>>> From: David Laight >>>>>>>> >>>>>>>> This lets the compiler inline it into import_iovec() generating >>>>>>>> much better code. >>>>>>>> >>>>>>>> Signed-off-by: David Laight >>>>>>>> Signed-off-by: Christoph Hellwig >>>>>>>> --- >>>>>>>> fs/read_write.c | 179 ------------------------------------------------ >>>>>>>> lib/iov_iter.c | 176 +++++++++++++++++++++++++++++++++++++++++++++++ >>>>>>>> 2 files changed, 176 insertions(+), 179 deletions(-) >>>>>>> >>>>>>> Strangely, this commit causes a regression in Linus's tree right now. >>>>>>> >>>>>>> I can't really figure out what the regression is, only that this commit >>>>>>> triggers a "large Android system binary" from working properly. There's >>>>>>> no kernel log messages anywhere, and I don't have any way to strace the >>>>>>> thing in the testing framework, so any hints that people can provide >>>>>>> would be most appreciated. >>>>>> >>>>>> It's a pure move - modulo changed line breaks in the argument lists >>>>>> the functions involved are identical before and after that (just checked >>>>>> that directly, by checking out the trees before and after, extracting two >>>>>> functions in question from fs/read_write.c and lib/iov_iter.c (before and >>>>>> after, resp.) and checking the diff between those. >>>>>> >>>>>> How certain is your bisection? >>>>> >>>>> The bisection is very reproducable. >>>>> >>>>> But, this looks now to be a compiler bug. I'm using the latest version >>>>> of clang and if I put "noinline" at the front of the function, >>>>> everything works. >>>> >>>> Well, the compiler can do more invasive optimizations when inlining. If >>>> you have buggy code that relies on some unspecified behavior, inlining >>>> can change the behavior ... but going over that code, there isn't too >>>> much action going on. At least nothing screamed at me. >>> >>> Apart from all the optimisations that get rid off the 'pass be reference' >>> parameters and strange conditional tests. >>> Plenty of scope for the compiler getting it wrong. >>> But nothing even vaguely illegal. >> >> Not the first time that people blame the compiler to then figure out >> that something else is wrong ... but maybe this time is different :) > > I agree, I hate to blame the compiler, that's almost never the real > problem, but this one sure "feels" like it. > > I'm running some more tests, trying to narrow things down as just adding > a "noinline" to the function that got moved here doesn't work on Linus's > tree at the moment because the function was split into multiple > functions. > > Give me a few hours... I might be wrong but a) import_iovec() uses: - unsigned nr_segs -> int - unsigned fast_segs -> int b) rw_copy_check_uvector() uses: - unsigned long nr_segs -> long - unsigned long fast_seg -> long So when calling rw_copy_check_uvector(), we have to zero-extend the registers used for passing the arguments. That's definitely done when calling the function explicitly. Maybe when inlining something is messed up? Just a thought ... -- Thanks, David / dhildenb